The term nanoscale generally refers to solid particles whose average particle diameter lies in the submicron range, i.e. less than 1 μm, or is even less than 0.5 μm (500 nm). Owing to their dimensions, nanoparticles have properties which sometimes differ fundamentally from the properties of the same substance when it is in a less fine distribution.
Nanoscale organic solids can be produced by various methods, in particular by grinding, reactions in the gas phase, reactions in a flame, by crystallization, precipitation, sol-gel processes, or by sublimation/desublimation.
It is known to carry out the desublimation by rapid relaxation of an inert gas jet, containing the starting substance for production of the nanoscale organic solid particles in molecularly disperse form, in a convergent nozzle whose narrowest cross section is configured so that the inert gas flow containing the solid particles in molecularly disperse form is accelerated to a velocity of Mach 1 or more. These are preferably so-called Laval nozzles, a nozzle shape in which a convergent input part is followed by a divergent output part.
For instance Streletzky in Journal of Chemical Physics, Vol. 116, No. 10, 2002, pages 4058 to 4061, describes controlled nucleation and controlled growth of nanoscale drops in supersonic nozzles, the attempt having been made to separate the nucleation zone from the growth zone. Relaxation of the free jet from the nozzle into a special expansion chamber, and cooling of the latter, are not mentioned.
In his Habilitationsschrift (qualifying postdoctoral thesis) “Erzeugung von organischen Nanopartikeln mit überkritischen Fluiden (“Production of organic nanoparticles with supercritical fluids”) at the TU Karlsruhe, 2001, pages 40 to 43 and 46 to 51, Michael Türk describes a method for producing organic nanoparticles by rapid expansion of supercritical solutions (RESS). Nozzles with diameters in the range of 50 μm are described, and the relaxation took place into an expansion chamber. The document contains no reference to influencing the reflux region in order to control the particle geometry.
Sane et al. in The Royal Society of Chemistry, 2003, 2720-2721, describe the production of porphyrin by the RESS method (Rapid Expansion of Supercritical Solutions). The RESS method utilizes the property of compressed gases that they dissolve substances. Carbon dioxide is mostly used for this. The nozzles used had a diameter of 50 μm and a diameter to length ratio of 1:4. The document gives no reference to an expansion chamber or influencing the reflux region.
The dissertation by Kodde (Hannover University, 1996) which is published in VDI-Fortschrittsberichte, series 3, No. 451, 1996, describes on pages 22 to 25, 84 and 85 studies of heterogeneous desublimation, a thermally conditioned gas mixture of air and succinic acid having been cooled by mixing with cold air in a tube flow. Owing to the cooling, the succinic acid desublimes on the submicronic particles contained in the cold air. In a continuation of this work by Wagner (cf. Chemie lngenieur Technik, Vol. 71, 1999), the mixture of carrier gas and succinic acid vapor was mixed with the gas flow containing nuclei. The desublimation was then initiated by relaxation of the gas mixture in a nozzle with a subsequent capillary. Heterogeneous condensation took place on the nuclei already present. None of the works describes expansion into an expansion chamber and influencing the reflux region in the expansion chamber.
Nanoparticulate organic solids are used in various fields. It is often necessary to comply with predetermined particle sizes, particle size distributions or particle shapes. Usually, these cannot be achieved by the particles initially formed during the expansion process.